The invention relates to an insulating tape for fabricating an insulating sleeve for electric conductors which is impregnated with a heat-hardening epoxy resin-acid anhydride hardener mixture. It is especially useful for the winding bars or coils of electric machines.
Insulating tapes which incorporate hardened epoxy resins are known, see for example U.S. Pat. No. 4,336,302. Also known are tapes with epoxy resins which incorporate bonding agent-accelerator systems. When the systems employ quaternary onium salts as accelerators, effective binding for most epoxy resin is provided. Furthermore, use of quaternary onium salts as accelerators will cause practically no ionic polymerization of epoxy compounds at room temperature. As a result, an insulating tape impregnated with an epoxy compound binder-quaternary onium accelerator can be stored at room temperature for up to more than three months without changing its properties. The quaternary onium salts also can cause polymerization of an epoxy resin-acid anhydride mixture at temperatures near 60.degree. C. and the polymerization time is substantially accelerated, so that it is possible to harden the portion of impregnating resin penetrating the insulating tape in an economically short time.
The bonding agent-accelerator system provided in this manner remains fully soluble in the impregnating resin under the conditions of drying and preheating as would be applied to insulating sleeves wound from insulating tapes prior to the impregnation of the sleeves with resin-hardener. There is no decrease in the hardening and accelerator reactivity relative to the reactivity of a combination of an impregnating resin and bonding agent accelerator system which is added to an insulating sleeve wound from tape unimpregnated with resin or the system.
According to present understanding, the known accelerators based upon quaternary onium salts are preserved in the system unchanged during the hardening of the impregnating resin and do not join the hardened bonding agent-impregnating resin body. Since they contain electrically charged particles, the possibility therefore exists that a field-oriented migration or alignment of these particles could occur due to the voltages applied to the insulating sleeves in operation. As a result, the insulation resistance would drop.
Although no indication has been observed so far in the insulating sleeves made with known bonding agent-accelerator mixtures and investigated as to such migration in a formed, hardened bonding agent-impregnating resin material, it is an object of the invention to prevent such a process from the start in an insulating tape of the type described above.